fix(appendix): use H2 for 附录七 to match sibling appendix files

The other appendix entries (附录四 best_practices, 附录五 debug,
附录六 resources, 附录八 learning_roadmap) all use H2. glossary.md
was the lone H1, which broke the table-of-contents nesting in
GitBook. Demoted to H2 to align.

appendix/glossary.md

@@ -1,4 +1,4 @@
-# 附录七：术语表
+## 附录七：术语表
 
 本附录整理了本书中常见的一些专业术语及其解释。